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Well, yes and no. When the letter y is used in place of an i for the phonetic, it is widely considered to be a vowel (hence it's sometimes amusingly referenced "the vaccilating vowel"). In phonics, the rules are simple:

1. The letter y is a consonant when it is the first letter of a syllable that has more than one letter.
2. If y is anywhere else in the syllable, it is a vowel.

Indeed - in such cases, w is considered to be a vowel because it is in the middle of a syllable, and represents a variant of the letter u.

One of the things that I learned when doing A Level English is that the letters a, e, i, o and u aren't always vowels as well. Vowels represent the phonic representation of the letter, and if the letter doesn't make the vowel sound, it's not a vowel at that point. What does this mean in practical effect? Well, it simplifies the case for knowing when to use "an" versus "a". If the phonic is a vowel sound, then you prefix with "an" instead of "a" - which makes it far easier to understand for people who mangle the language trying to talk about "I went to an university", when they should be using "I went to a university" because the U isn't a phonic vowel.